add ssh key passphrase support?

[gmichalec-pandora]

Thanks for this library!
Would it be possible to support encoding the ssh key with a passphrase?

[paulmillr]

No plans on coding that by ourselves, but we'll consider merging contributions.

I'm concerned about bloat. For example, there are likely different encryption algorithms. Right? We can't be asked to support something as old as DES. Does it make sense to only support chacha? Is chacha even supported by specs? What specs are there? Etc.